1. Field of the Invention
The present invention relates to a microlithography objective for EUV lithography with a wavelength in the range of 10-30 nm for imaging an object field in an image field. The invention also provides for a projection exposure system and a chip manufacturing process.
2. Description of the Prior Art
Lithography with wavelengths  less than 193 nm, particularly EUV lithography with xcex=11 nm or xcex=13 nm are discussed as possible techniques for imaging of structures  less than 130 nm, most preferably  less than 100 nm. The resolution of a lithographic system is given by the following equation:
RES=k1xc2x7xcex/NA,
where k1, denotes a specific parameter of the lithography process, xcex denotes the wavelength of the incident light and NA denotes the numerical aperture of the system on the image side.
For imaging systems in the EUV range, reflective systems with multiple layers are used substantially as optical components. Preferably, Mo/Be systems are used as multilayer systems for xcex=11 nm and Mo/Si systems are used for xcex=13 nm.
The reflectivity of the multilayer systems used currently lies in the range of approximately 70%. Therefore a projection objective for EUV microlithography should have has as few optical components as possible, in order to achieve a sufficient light intensity.
In order to achieve a resolution that is as high as possible, on the other hand, it is necessary that the system have an aperture that is as large as possible on the image side.
For lithography systems it is advantageous if the beam path within the projection objective is free of obscurations. Projection objectives should have no mirrors with transmissive areas, especially openings, since transmissive areas lead to shading. If an objective has no mirrors with transmissive areas, the objective has an obscuration-free beam path and the exit pupil of the objective is free of shading and free of obscurations. Furthermore, the aperture diaphragm of such an objective does not need to have a shading device. A disadvantage of a system with an exit pupil being shaded, e.g., a so-called Schwarzchild mirror system, is that structures of a specific size can be imaged only with restrictions. The exit pupil is defined as the image of the aperture diaphragm formed by the optical elements arranged in the beam of the objective between the aperture diaphragm and the image plane.
4-Mirror systems for microlithography have become known, for example, from U.S. Pat. No. 5,315,629 or EP 0 480,617 B1. Such systems, however, permit a numerical aperture only of NA=0.1 on the image side with a sufficient field size of at least 1.0 mm scanning slit width. The limit of resolution lies in the range of 70 nm with the use of x-ray light with a wavelength of 10 to 30 nm.
6-Mirror systems for microlithography have been made known from the publications U.S. Pat. No. 5,153,898; EP-A- 0 252,734; EP-A-0 947,882; U.S. Pat. No. 5,686,728; EP 0 779,528; U.S. Pat. No. 5,815,310; WO 99/57606; and U.S. Pat. No. 6,033,079.
Such 6-mirror systems have a numerical aperture  less than 0.3 on the image side, which leads to a resolution limit in the range of 30 nm with the use of x-ray light with a wavelength of 10-30 nm.
Another disadvantage of both 4-mirror and 6-mirror systems is the fact that there are only a few possibilities for correction of imaging errors.
A microlithography projection objective with eight mirrors has become known from U.S. Pat. No. 5,686,728. This projection objective has a high numerical aperture of NA=0.55 on the image side. Of course, the projection objective known from U.S. Pat. No. 5,686,728 is suitable only for wavelengths longer than 126 nm, since, for example, the angle of incidence of the chief ray of the field point, which lies on the axis of symmetry in the center of the object field is so large that this 8-mirror system cannot be operated in the EUV wavelength region from 10 to 30 mn. Another disadvantage of the system according to U.S. Pat. No. 5,686,728 is that all eight mirrors are formed aspheric and that the angle of the chief ray at the object has a value of 13xc2x0 with a numerical aperture of 0.11 on the object side.
A first object of the invention is to provide a suitable projection objective for lithography with short EUV wavelengths in the range of 10 to 30 nm, which is characterized by a large numerical aperture and improved possibilities of imaging correction when compared with previously known projection systems for EUV microlithography.
Another object of the invention is to provide a microlithography projection objective for lithography with wavelengths xe2x89xa6193 nm, which has both a large aperture and which can be manufactured in a simple manner.
According to the invention, the first object is solved by a microlithography projection objective for EUV lithography with a wavelength in the range of 10-30 nm in that the microlithography projection objective has eight mirrors instead of four or six mirrors.
The inventors have recognized surprisingly that such an objective makes available both a sufficient light intensity as well as a sufficiently large numerical aperture in order to meet the requirements for high resolution. Furthermore such an objective provides sufficient possibilities for imaging correction.
In order to achieve a resolution as high as possible, in an advantageous embodiment, the numerical aperture of the projection objective on the image side is greater than 0.2.
In order to minimize the angle of incidence of the chief ray of the field point, which lies on the axis of symmetry and in the center of the object field, the numerical aperture on the image side of the projection system according to the invention is advantageously limited to NA less than 0.5.
In order to force a ray bundle, or light bundle, in the direction of the optical axis (HA) and to avoid off-axis segments of the mirrors having a large distance to the optical axis (HA) in a particularly advantageous embodiment the projection objective is designed in such a way that at least one intermediate image of the object field is formed in the beam path of the rays of the projection objective between the object field and the image field.
In the present application, that part of the mirror on which the light rays that are guided through the projection objective impinge is denoted as the off-axis segment of the mirror. The distance of the off-axis segment from the optical axis (HA) in the present application is the distance of the point of incidence of the chief ray of the central field point onto the segment of the mirror from the optical axis (HA).
In order to minimize the angle of incidence on the first mirror of the projection objective according to the invention, in a particularly advantageous embodiment of the invention, a diaphragm, which is preferably circular or nearly circular, is arranged in the light path between first and third mirrors, preferably on or in the vicinity of the first mirror, or on or in the vicinity of the second mirror. xe2x80x9cIn the vicinityxe2x80x9d in the present application is understood as the distance of the diaphragm from the closest mirror that is less than {fraction (1/10)}th the distance from the preceding mirror to the mirror in the vicinity of the diaphragm. For example, xe2x80x9cin the vicinity of S2xe2x80x9d means that the following applies:
{overscore (BS)}2xe2x89xa6{fraction (1/10)}{overscore (S1S2)}
Wherein {overscore (BS)}2 denotes the distance of the diaphragm to the second mirror and {overscore (S1S2)} denotes the distance between the first and second mirror. Such an arrangement permits a minimal separation of the ray bundle in the front part of the objective. This means that the angles of incidence on the first, second and third mirrors are reduced. In addition, such an arrangement of the diaphragm yields a configuration where the off-axis segment of the third mirror lies directly below the optical axis and is nearly a mirror image of the off-axis segment of the first mirror S1. Furthermore, the angles of incidence on the fourth and fifth mirrors are reduced, since the distance of the ray bundle from the optical axis between the fourth and fifth mirrors is minimal.
In order to produce small angles of incidence on the mirrors, it is further of advantage, if the distances between the off-axis segments of the mirrors and the optical axis (HA) are kept small. Since these distances can be varied randomly by an appropriate scaling, they are characterized by their size ratio relative to the structural length of the objective in the present application. It is particularly advantageous, if the following relation is fulfilled:
the distance of the off-axis segment of each mirror to the optical axis less than 0.3 times structural length of the objective and preferably:
the distance of the off-axis segment of each mirror to the optical axis less than 0.25 times structural length of the objective.
With the aforementioned relation, small angles of incidences are produced.
In a further form of embodiment of the invention, the radius of curvature of at least one mirror is larger than the structural length of the projection objective. The distance from the object to be imaged up to its image is understood as the structural length of the system in the present application. It is particularly advantageous that the aforementioned condition for the radius of curvature applies to the second, third and fourth mirrors so that the paths of the ray bundles from the first to the second mirrors and from the third to the fourth mirrors are nearly parallel. With such a configuration, a minimal separation of the ray bundles and large drift paths is achieved. In the present application, the distance between the vertexes of two consecutive mirrors in the light path is to be understood as the drift path. The aforementioned conditions contribute to small angles of incidence on the mirrors.
The projection objective can be further characterized from the sum of (a) the lengths of all drift paths between pairs of sequential mirrors in said light path, (b) the length from said object plane to a vertex of said first mirror (S1) in said light path, and (c) the length from a vertex of said eighth mirror (S8) to said image plane in said light path, as indicated in the following formula:                     sum        =                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            drift            ⁢                          xe2x80x83                        ⁢                          path              S1S2                                +                                                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            drift            ⁢                          xe2x80x83                        ⁢                          path              S2S3                                +                                                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            drift            ⁢                          xe2x80x83                        ⁢                          path              S3S4                                +                                                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            drift            ⁢                          xe2x80x83                        ⁢                          path              S4S5                                +                                                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            drift            ⁢                          xe2x80x83                        ⁢                          path              S5S6                                +                                                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            drift            ⁢                          xe2x80x83                        ⁢                          path              S6S7                                +                                                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            drift            ⁢                          xe2x80x83                        ⁢                          path              S7S8                                +                                                  xe2x80x83                ⁢                              length            ⁢                          xe2x80x83                        ⁢            from            ⁢                          xe2x80x83                        ⁢            object            ⁢                          xe2x80x83                        ⁢            plane            ⁢                          xe2x80x83                        ⁢            to            ⁢                          xe2x80x83                        ⁢                          vertex              S1                                +                                                  xe2x80x83                ⁢                  length          ⁢                      xe2x80x83                    ⁢          from          ⁢                      xe2x80x83                    ⁢                      vertex            S8                    ⁢                      xe2x80x83                    ⁢          to          ⁢                      xe2x80x83                    ⁢          image          ⁢                      xe2x80x83                    ⁢                      plane            .                              
In a preferred embodiment, this sum is at least 2.5 times the structural length of the projection objective. The projection objective is preferably further characterized in that at least one drift path is longer than 70% of the structural length of the projection objective.
In another embodiment of the invention, the microlithography projection objective is designed in such a way, that the angle of the chief ray at the object is smaller than twice the value of the object-side aperture (NAO). This is an advantage, since shading effects on a mask are reduced thereby.
It is a particular advantage if the projection objective has two intermediate images. The first intermediate image in a system with two intermediate images is formed preferably between the second and third mirror. This leads to the fact that the first, second, third and fourth mirrors have off-axis segments in the vicinity of the axis. In order to assure off-axis segments near to the axis for as many mirrors as possible in the objective part comprising the fifth, sixth, seventh and eighth mirrors, it is advantageous that the projection objective is designed in such a way that the second intermediate image is formed in the path of the rays between the sixth and seventh mirrors. It is particularly preferred, if the angle of incidence of the chief ray of the field point, which lies on the axis of symmetry in the center of the object field, is smaller than 20xc2x0 on all mirrors, in the case of a system with two intermediate images.
In a preferred embodiment with two intermediate images, at least one of the eight mirror surfaces is formed spherical.
It is particularly advantageous if those mirrors of the objective with the largest distance of the off-axis segment from the optical axis is/are formed spherical, since the interferometric testability of off-axis aspheric profiles becomes difficult with an off-axis segment having a large distance to the optical axis. In general the interferometric testability becomes more difficult if the distance between the off-axis segment and the optical axis becomes larger.
In a system with two intermediate images between the second and third mirror as well as between the sixth and seventh mirror, the sixth mirror is the mirror with the off-axis segment with the largest distance from the optical axis. In such an embodiment, preferably the sixth mirror is formed spherical for the sake of interferometric testability.
In addition to the projection objective, the invention also provides a projection exposure system, wherein the projection exposure system comprises an illumination device for illuminating a ring field as well as a projection objective according to the invention.